We plan to continue to assess the usefulness of terbium ion as a fluorescent probe for studying reaction kinetics in ribosome solutions by the stop-flow method. We plan to build and put into operation for studying the fast interaction process between ribisomal subunits a 90-degree light-scattering adaptation of the pressure-jump shock-tube apparatus developed by Hoffmann and yeager with a conductivity minor for studying kinetics in ionic micelles. With such an instrument, we will be able to extend our observation time for pressure-jump experiments down to the range of several microseconds. This will make it possible for us to resolve relaxation times with this technique from several microseconds up to several minutes, since our present apparatus is effective between several milliseconds and several minutes. We expect to be able to resolve the fast relaxation time heretofore resolved only with the aid of the temperature-jump apparatus in our laboratory. We have noticed that addition into several ml. of standard buffers containing from 2.5 mM to 5 mM Mg ions of only microliters of ribosomal suspensions equilibrated with 10 mM Mg ions cause unexpectedly large change in Mg ions concentration. This suggests a large buffering effect by the ribosomes themselves. The standard buffers used in ribosome work are good pH buffers, but have really no buffering capacity for Mg ions. It is possible that the standard methods of ribosome analysis on sucrose density gradients have been troubled by artifactual problems generated by gradients of Mg ions in the sedimentation column, because of the lack of buffering capacity for Mg ions in the buffer itself. It is known that whole ribosomes and their subunits do not have identical binding capacities for Mg ions, which would suggest that such gradients are developed. We will try the effects of adding good Mg ions buffers.